Top drives are used to suspend and rotate a string of drill pipe and/or casing in drilling applications. The top drive is supported by a drilling line wrapped on a set of sheaves and connected to drawworks at one end and a deadline anchor at the other end. The top drive supports the drill string via a thrust bearing. Mud may be pumped into the drill string via a swivel. Furthermore, the top drive generally includes one or more motors (electric or hydraulic) which generate(s) the rotation of the drill string. The reaction torque applied to the top drive may be transmitted to the mast via a top drive carriage that rides on mast guide rails.
Various measurements may be taken to manage the drilling process, including measurements of top drive conditions. Hookload and hook elevation above the rig floor are two such measurements. These measurements may be employed to calculate drilling parameters such as weight on bit (WOB), rate of penetration (ROP), and depth. A variety of other types of measurements are used to calculate these and other drilling parameters.
Sensors are distributed on the drilling rig to measure conditions directly or indirectly. Accordingly, non-linearities, reduced resolution, noise, etc. may be part of the measurement system and may or may not be corrected in the measurements. For example, hook load has been measured indirectly on the “dead-line” of the drilling line, near or on the anchor below the rig floor. Hookload has been measured by using hydraulic load cells at the dead line anchor. Generally, there is no correction for friction in the sheaves in this measurement. In addition, the weight of the travelling block, hook and top drive may limit the resolution of the hook-load measurement.
The use of a hydraulic load cell at the dead line anchor is illustrated in U.S. Pat. No. 6,918,454. A load sensing device, such as a strain gage or a hydraulic load cell is affixed to the dead line and produces an output control signal indicating the tension in the dead line and consequently, the load carried by the traveling block or pull on the drill bit (POB). This POB measurement from the load sensing device is sent from the strain gage to the control system. Various tension measuring devices may be employed to indicate the tension conditions on the dead line. The actual hook load or POB may be calculated using the load sensing device input in conjunction with the number of lines strung and a calibration factor. Alternatively, a conventional load cell, hydraulic tension transducers or other load measuring device may be associated with the derrick to provide the output control signal representative of the load carried by the traveling block.
A more direct hookload measurement has been taken by installing measurement and communication devices on top of the drill string. For example, hydraulic load cells have been installed at the crown block. Alternatively, hookload measurement devices rotate with the drill string. When such devices are installed, communication between these systems attached to (and rotating with) the drill string to the rig system may be performed either by rotary transformer (inductive coupling), or sliding contacts or e-mag communication (such as WIFI). The main node is thus at the rig, resulting in long communication paths form the rotating devices with risk of signal corruption.
More recently, modern rigs (mainly offshore) have used top drives with built-in horizontal-mounted load cell pins positioned between the travelling blocks and the top drives. Other hookload measuring systems have load cells installed in pins on the crown block, load cells installed in the deadline, or strain measurement sensors installed on the Steel Wire Rope (SWR).
A horizontal mounted load cell pin is illustrated in US Publication No. 2016/0245727. One or more strain gauge sensors are located in each load bearing clevis pin required to lock the crown block into its position. One conventional installation includes four load pins providing four load measurements. For an accurate measurement in a marine environment all four sensors need to be operational as the load distribution across the four pins is not expected to be homogenous.
Due to various reasons, the accuracy of such load cells have proven to have significant deviation from real load. It is a fact that monitoring load via horizontal load pins that monitor shear forces by design have limitations regarding accuracy. Thus, more accurate devices are needed to monitor the weight of tubular and ancillaries lowered and/or hoisted out from a drilling well.